cell division/ regulation

Question 1

What is the Cell Cycle, what are its phases?

Answer 1

process cells go through to replicate-only some cells
interphase:
G(gap)1-resting/normal job-in this phase most of time
S - DNA synthesis
G2 -
mitosis - nuclear division
cytokinesis - cytoplasm division

Question 2

What is the significance of the key events that occur during Mitosis?

Answer 2

G2 of interphase:
chromosomes condense
nuclear envelope dissolves
centrosomes develop

prophase:
nuclear membrane disintegrates
Chromosome fully condenses (visible under microscope)
centrosomes replicate+spindles form

prometaphase:
spindle formed
chromosomes fully condensed - attach to spindles
no nucleus membrane

metaphase:
chromosomes align at equator-attach via centromere
(mitosis)line up independently but as duplicate chromatids

anaphase:
spindles contract
chromatids separate

telophase:
chromosomes reach poles of cells(spindles shorten/contracting)
nucleus reformes
spindles regrade

Question 3

How are separate chromatids held together?

Answer 3

Chromosomes are JOINED by molecule COHESIN
Multi subunit protein
Forms a loop between subunits
Cohesin forms a dimer loop
Two helices of DNA captured inside the loop

Question 4

How do the chromosomes condense?

Answer 4

DNA condensation caused by multiple rounds of folding and coiling
Chromosomes are CONDENSED by CONDENSIN
Final chromosome condensation by CONDENSIN
Multi-subunit Dimerises
Diners loop around DNA, securing loops
DNA loop through the neck of the ring
Structural maintenance of chromosomes (SMC) complexes
Coordinated activity of Cohesin and Condensin enables the chromatids to condense into 300nm fibres and looped domains

Cohesin and Condensin are very similar in structure
Cohesin and Condensin are descended from the same bacterial protein
They work together

Question 5

How do chromosomes attach to the Spindle?

Answer 5

Microtubules form a network,
connecting poles of the cell with each other and the cell membrane
Binds to cromosomes
Facilitates chromosome movement
Microtubules produced by the CENTRIOLES
2 Centrioles in a CENTROSOME
Tubulin molecules become polymerised
into spirals to form microtubules

Question 6

What orientates the chromosomes? - ensuring one chromatid goes to each side

Answer 6

Chromosomes are orientated so that one chromatid faces each pole of the spindle
the kinetochore and kinesins enable the chromosomes to ‘walk’ to the ends of a spindle fibre

Question 7

What pulls the chromatids apart in Anaphase?

Answer 7

SEPARASE allows chromatids to separate

interpolar microtubules start to dissociate from each other and break down/condence
kinetochore microtubules get shorter
pulls chromatids away from centre
the separase-‘snips’ the cohesin, thats keeping them attached

Question 8

Prophase 1

Answer 8

Nuclear membrane starts to break down
Chromosomes start to condense
Centrosomes (made of 2 centrioles) replicate and move to pole of cell, start forming spindle
C1 from mum and C1 from dad Line up with each other(C2 and C2 etc.) and join to spindle and cross over

1. Chromosomes condense (Equivalent to the processes in Prophase in Mitosis)
2. Chromosomes align with homologous pair
   -Synaptonemal complex begins to form, joining homologous chromosomes together along the length of the chromatids
3. Chromosomes aligned fully in homologous pairs
   Synapsis complete – chromatids joined along lengths
   Crossover events occur by breakage of the DNA and re- fusion with alternate chromatid
4. Synaptonemal complex dissolves, leaving chromatids free.
   Homologous Chromosomes still joined together, but only via crossover junctions
   Bivalents/Tetrads visible under microscope
5. Bivalents associate with the meiotic spindle
   Orientate in HOMOLOGOUS PAIRS at the equator of the cell
   One homologue attached to each spindle pole

Question 9

Metaphase 1

Answer 9

HOMOLOGOUS PAIRS line up at the equator (still in Bivalents)
ONE kinetochore per chromosome attached to spindle.
This is the point of meiosis at which most egg cells are suspended in humans

Question 10

Anaphase 1

Answer 10

Replicated pairs of CHROMATIDS (i.e. individual chromosomes) move towards the poles of the cell.

Question 11

Telophase 1

Answer 11

Nuclear membrane begins to reform
Chromosomes decondense

Question 12

Prophase 2

Answer 12

chromosomes condense and attach to spindle

Question 13

Metaphase 2

Answer 13

chromosomes line up at equator of cell
each chromatid is attached to the spindle

Question 14

anaphase 2

Answer 14

chromosomes pulled apart
chromatids separate and are pulled to opposite ends of the cell

Question 15

telophase 2

Answer 15

chromosomes de-condence, nuclear membrane reforms, cytokinesis occurs next

Question 16

crossing over

Answer 16

Crossover is facilitated by the SYNAPTONEMAL COMPLEX, a collection of proteins and chromosomal structures, acting like a zip to bring the chromatids together

LATERAL ELEMENTS and CENTRAL ELEMENTS bind the chromatids of homologous chromosomes together. Draw DNA strands close and enables breaks in DNA

Bivalents are not simple, Larger chromosomes have more crossover events
Homologous chromosomes separate during Anaphase I. BUT CHROMATIDS REMAIN ATTACHED TOGETHER

Crossovers enable complete transferral of DNA regions from one chromosome to another
Mixes up DNA from either parent.

Question 17

mitosis V meiosis

Answer 17

mitosis:
2 cells produced
genetically identical to parent
genetically identical to each other
daughter cells 2n / 2c

meiosis:
4 cells produced
genetically different from parent
genetically different from one another
daughter cells n / c

Question 18

cytokinesis